1. Field of the Invention
The present invention relates generally to video tape recorders and more particularly is directed to a surveillance or time-lapse video tape recorder.
2. Description of the Prior Art
Surveillance video tape recoders (SVTRs) are used for security or other purposes to provide a video recording of events occurring over extended periods of time. Although conventional magnetic tapes are designed to provide a maximum recording time of 3-4 hours, SVTRs are intended to record events over a much longer total elapsed time, for example, for as long as the 16 hours when a business may be closed. Such ability to record the events of 16 hours on a magnetic tape normally adapted to hold only 3-4 hours of recording, is achieved by time-lapse recording of still images.
In other words, instead of making a real time recording in which the time required for reproduction is equal to the elapsed time of events during recording, a time-lapse recording is made in which individual frames of a video signal corresponding to respective individual images are recorded at spaced time intervals in respective pairs of tracks on a magnetic tape by respective rotary magnetic heads. To save space on the tape, the tape is held stationary during the actual recording in each track, moved to a new position in anticipation of the recording of the next frame and thereafter held stationery until the recording of a subsequent frame has been completed. Upon reproduction, the tape may be driven at a constant speed in a continuous playback or search mode to locate a particular image of interest to the viewer, for example, an image showing the face of a suspected burglar, and then the single frame corresponding to that image may be repeatedly reproduced with the tape held stationary to provide a still image of good resolution. The foregoing system provides the necessary information for security purposes while saving significant amounts of tape.
However, the slant angle of the tracks in which signals are recorded while the tape is stationary or at rest is different from the angle of the path of the rotary magnetic heads as they scan across the tape while it is being moved in the continuous playback or search mode. Consequently, each path traced by a rotary magnetic head across the tape in the continuous playback or search mode will intersect two or more of the tracks in while the signals were recorded by heads having different azimuth angles. Such scanning by the heads along paths at an angle to the tracks causes noise bars to appear in the reproduced images in the continuous playback or search mode.
It has been proposed to control the tracking in the search mode so as to render the noise bars relatively inconspicuous. One system proposed for providing the requisite tracking includes a control (CTL) head to detect the position of a control signal recorded in a longitudinal track on the tape and, on the basis thereof, to correspondingly control a capstan motor driving the tape. Other systems for rendering the noise bar inconspicuous employ a video tape recorder having a four-head arrangement, or use a guard-band system in which guard bands are provided between the successive tracks and signals are recorded and reproduced by two heads having the same azimuth angle. However, the four-head arrangement is relatively complicated and expensive, while the guard-band system is not compatible with present standard video tape recorders. Moreover, the video tape recorder using the CTL tracking system requires a special CTL head and also requires careful and time consuming adjustment thereof during the manufacturing of the video tape recorder in order to achieve the necessary tracking control for rendering the noise bars inconspicuous.